This oil spill in the Gulf of Mexico has been in the news a lot lately. Recently, I came across this satellite photograph from NASA, which shows the oil heading southeast in a long stream. There’s a lot of speculation on what will happen in the days to come if the spill isn’t capped soon. Where will the oil go? Which areas will be affected?
Here’s a map showing the topology of the sea floor where the Deepwater Horizon was located. As you can see, the site is near a steep incline, where the continental shelf drops off sharply towards the sea floor. The depth of the sea floor was around 5000 feet.
I’ve come across a bunch of comments at various websites, with some people saying “oh well, the oceans are huge, so what if we have a spill? If you consider the amount of oil compared to the vast volume of the oceans, it is so tiny it doesn’t matter”. This kind of talk seems singularly uninformed at best and deliberate distortion of the facts at worst to me. It’s clearly obvious if you follow the news at all, that the spill isn’t being distributed evenly among the oceans. Tar balls and oil are appearing on the Louisiana coast. And as the satellite picture from NASA shows, the oil is definitely being channeled in a very specific direction. So I thought to write this note to explain what is happening, and what we might expect in the days to come.
In order to understand what’s happening to the oil as it leaks out, we need to understand water currents and wind directions in the Gulf of Mexico. These can be summarized as follows:
- There is a deep water current that enters the Gulf of Mexico from the south, loops through the Gulf, and then exits through the Florida Strait and curves northwards along the east coast of the US, as the Gulf Stream.
- The prevailing winds in this area are the Westerlies, or anti-trade winds, which blow from south west to north east. Winds do not affect deep waters, but they do in fact produce currents in surface waters. It might seem intuitively right that the surface water current should be in the same direction as the wind, but this is not so. In fact, surface water currents flow in a direction 90 degrees to the right of the wind direction, because of a phenomenon known as Eckman Transport. Since the prevailing winds in this region are southwest to northeast, surface currents induced by the winds flow northwest to southeast.
- The movement of the oil itself is subject to the Coriolis Effect. This is an effect which happens due to the west to east rotation of the Earth. Air or water or oil flowing in a fluid medium is affected in that its path curves westwards if the flow is towards the equator, and curves eastwards if the flow is away from the equator.
- The oil leak is occurring at the bottom of the ocean. The oil rig collapsed to the sea floor. The leaks are in the valve at the sea bed, and also in the riser pipe which is now lying collapsed on the sea floor. So the oil is being released at the sea floor, and then rising slowly to the surface. Therefore, since this oil is traversing the entire depth of the sea, from the sea floor to the surface, it is affected by both deep water and surface currents.
You can see these things in this diagram I made:
Notice the deep water current displayed in tan. This current enters the Gulf of Mexico from the south, loops around the Gulf, and then exits around Florida, only to swing back north and proceed along the eastern seaboard as the Gulf Stream.
Next, consider the winds, shown in yellow on the map. These are the Westerlies, or anti-trade winds. At these latitudes, they blow from southwest to northeast (as an aside, this is why “weather” in the form of storms tends to move from southwest to northeast in the continental US). Because of these winds, the movement of surface water is from northwest to southeast, as shown by the orange arrow. This is because of Eckman Transport, which tends to push water 90 degrees to the right of the wind direction. Remember, this represents surface water only, since the wind does not affect deep water.
Now in the light of these factors, look at how this oil slick has evolved over time:
These are four satellite images taken by NASA, from May 9th, 10th, 11th and 17th. Note that the images have not been equally scaled. Specifically, I zoomed out on the May 17th image to include the long tail.
As you can see, the slick slowly elongates in a north-south direction, and develops a “tail” pointing south. Between May 11 and May 17, the tail grows enormously, curving southeast. What could explain this effect?
If you look at the diagram I made earlier, you can see that the slick was slightly north of the loop current. The position of the loop current isn’t fixed, it has some daily/weekly variation. During early May, satellite imaging showed that the northernmost extent of the loop current was about 50 miles south of the oil spill, though the strongest currents were about 80 miles south. Since then, as we’ve seen in the satellite maps, the spill has extended far southwards. The satellite image from May 17th shows the slick extending 100+ miles south of its origin. This would put it well within the range of loop current, even if the loop current has shifted somewhat in this time.
So far, BP is still saying that the oil has not entered the loop current. There is still some doubt about this, but some scientists think it may already have entered the loop circulation. We will probably know for sure in a day or two.
Here is how I think the slick has proceeded:
- Oil leaking from the sea floor rose up and encountered local currents. Since the leak origin was fairly close to the shore where currents are quite turbulent, it initially spread pretty randomly, forming a large patch offshore.
- As oil continued to pump out from the leak and the volume of the leak increased, some of it drifted southwards. It acquired a slight easterly curve, because surface currents move in a direction 90 degrees to the right of the prevailing winds, due to Eckman Transport. The prevailing winds in this area are the anti-trade winds, or westerlies, which blow from southwest to northeast.
- This could very well explain the southeastward trajectory of the slick, without needing to invoke the loop current. However, the fact that it’s spread so far south and is either in or very close to loop current trajectory, means that there is a good chance that at least the tail end of it is being pulled along by the loop current, which would carry it eastwards at this point. The sudden sharp bend it makes at the very southern end also suggests that something else happened at this point – perhaps that something else was the slick being drawn into the loop current.
If the slick hasn’t been drawn into the loop current, it seems almost certain that it will, since it’s so very close.
Future Expectations
The size of the spill is under debate. BP has stated that it’s about 5000 barrels per day, but independent estimates from other scientists say that it could be as much as 70,000 barrels per day. The fate of the spill depends upon its size, which means how much oil is being spilled per day, and how many days it continues to flow before it’s capped.
If a substantial amount of oil leaks out, where will it go? As you can see in the maps, the deep water currents loop around the Gulf, and head north along the Gulf Stream, which passes by the east coast of the US. Oil that gets caught in this current and doesn’t make it to the surface until much later may therefore follow this path and only surface off the east coast of Georgia or the Carolinas. But this would take a lot of oil, and perhaps there won’t be so much.
The more immediate danger is to the Florida Keys and the Bahamas, as well as the north coast of Cuba. Because the loop current stays pretty far away from the west coast of Florida, it seems unlikely that much oil could end up there. But it comes very close to land when the current passes out of the Florida Strait. It has to pass through a triangle formed by the Keys to the north, Cuba to the south, and the Bahamas to the east. Those are the likely danger spots.
If there are strong winds in this period, driving strong surface currents, then the Bahamas and Cuba are probably even worse off, since wind-driven surface currents will be in a southeasterly direction. By the same token, if the oil makes it into the Gulf Stream and heads up the east coast of the US, winds may cause the coasts of Georgia and the Carolinas to be spared, since surface currents would push the oil away from the shores.
It’s also important to remember that the paths of these currents and even the wind directions aren’t always constant. There is day to day variation, as well as seasonal variation now that summer is coming. So these predictions are only approximate.
I also wrote a short article on ocean currents which you can read here, if you’re interested.
EDIT: There were some news stories recently that tar balls have washed up in the Florida Keys. These are not from the Deepwater Horizon spill. As you can see from the maps, the spill hasn’t reached anywhere near the Florida Keys so far. The Gulf of Mexico has a lot of oil rigs and many spills have happened here before, so it’s not uncommon for tar balls to show up on any of these coasts.
